Memory device comprising electrically floating body transistor

ABSTRACT

A semiconductor memory instance is provided that includes an array of memory cells. The array includes a plurality of semiconductor memory cells arranged in at least one column and at least one row. Each of the semiconductor memory cells includes a floating body region configured to be charged to a level indicative of a state of the memory cell. Further includes are a plurality of buried well regions, wherein each of the buried well regions can be individually selected, and a decoder circuit to select at least one of the buried well regions.

CROSS-REFERENCE

This application is a continuation of co-pending U.S. application Ser.No. 14/825,628, filed Aug. 13, 2015, which claims the benefit of U.S.Provisional Application Nos. 62/038,188, filed Aug. 15, 2014;62/051,759, filed Sep. 17, 2014; and 62/058,892, filed Oct. 2, 2014,each of which applications are hereby incorporated herein, in theirentireties, by reference thereto.

FIELD OF THE INVENTION

The present invention relates to semiconductor memory technology. Morespecifically, the present invention relates to a semiconductor memorydevice comprising an electrically floating body transistor.

BACKGROUND OF THE INVENTION

Semiconductor memory devices are used extensively to store data. Memorydevices can be characterized according to two general types: volatileand non-volatile. Volatile memory devices such as static random accessmemory (SRAM) and dynamic random access memory (DRAM) lose data that isstored therein when power is not continuously supplied thereto.

A DRAM cell without a capacitor has been investigated previously. Suchmemory eliminates the capacitor used in the conventional 1T/1C memorycell, and thus is easier to scale to smaller feature size. In addition,such memory allows for a smaller cell size compared to the conventional1T/1C memory cell. Chatterjee et al. have proposed a Taper Isolated DRAMcell concept in “Taper Isolated Dynamic Gain RAM Cell”, P. K. Chatterjeeet al., pp. 698-699, International Electron Devices Meeting, 1978(“Chatterjee-1”), “Circuit Optimization of the Taper Isolated DynamicGain RAM Cell for VLSI Memories”, P. K. Chatterjee et al., pp. 22-23,IEEE International Solid-State Circuits Conference, February 1979(“Chatterjee-2”), and “dRAM Design Using the Taper-Isolated Dynamic RAMCell”, J. E. Leiss et al., pp. 337-344, IEEE Journal of Solid-StateCircuits, vol. SC-17, no. 2, April 1982 (“Leiss”), all of which arehereby incorporated herein, in their entireties, by reference thereto.The holes are stored in a local potential minimum, which looks like abowling alley, where a potential barrier for stored holes is provided.The channel region of the Taper Isolated DRAM cell contains a deepn-type implant and a shallow p-type implant. As shown in “A Survey ofHigh-Density Dynamic RAM Cell Concepts”, P. K. Chatterjee et al., pp.827-839, IEEE Transactions on Electron Devices, vol. ED-26, no. 6, June1979 (“Chatterjee-3”), which is hereby incorporated herein, in itsentirety, by reference thereto, the deep n-type implant isolates theshallow p-type implant and connects the n-type source and drain regions.

Terada et al. have proposed a Capacitance Coupling (CC) cell in “A NewVLSI Memory Cell Using Capacitance Coupling (CC) Cell”, K. Terada etal., pp. 1319-1324, IEEE Transactions on Electron Devices, vol. ED-31,no. 9, September 1984 (“Terada”), while Erb has proposed StratifiedCharge Memory in “Stratified Charge Memory”, D. M. Erb, pp. 24-25, IEEEInternational Solid-State Circuits Conference, February 1978 (“Erb”),both of which are hereby incorporated herein, in their entireties, byreference thereto.

DRAM based on the electrically floating body effect has been proposedboth in silicon-on-insulator (SOD substrate (see for example “TheMultistable Charge-Controlled Memory Effect in SDI Transistors at LowTemperatures”, Tack et al., pp. 1373-1382, IEEE Transactions on ElectronDevices, vol. 37, May 1990 (“Tack”), “A Capacitor-less 1T-DRAM Cell”, S.Okhonin et al., pp. 85-87, IEEE Electron Device Letters, vol. 23, no. 2,February 2002 and “Memory Design Using One-Transistor Gain Cell on SOI”,T. Ohsawa et al., pp. 152-153, Tech. Digest, 2002 IEEE InternationalSolid-State Circuits Conference, February 2002, (all of which are herebyincorporated herein, in their entireties, by reference thereto) and inbulk silicon (see for example “A one transistor cell on bulk substrate(1T-Bulk) for low-cost and high density eDRAM”, R. Ranica et al., pp.128-129, Digest of Technical Papers, 2004 Symposium on VLSI Technology,June 2004 (“Ranica-1”), “Scaled 1T-Bulk Devices Built with CMOS 90 nmTechnology for Low-Cost eDRAM Applications”, R. Ranica et al., 2005Symposium on VLSI Technology, Digest of Technical Papers (“Ranica-2”),“Further Insight Into the Physics and Modeling of Floating-BodyCapacitorless DRAMs”, A. Villaret et al, pp. 2447-2454, IEEETransactions on Electron Devices, vol. 52, no. 11, November 2005(“Villaret”), “Simulation of intrinsic bipolar transistor mechanisms forfuture capacitor-less eDRAM on bulk substrate”, R. Pulicani et al., pp.966-969, 2010 17th IEEE International Conference on Electronics,Circuits, and Systems (ICECS) (“Pulicani”), which are herebyincorporated herein, in their entireties, by reference thereto).

Widjaj a and Or-Bach describes a bi-stable SRAM cell incorporating afloating body transistor, where more than one stable state exists foreach memory cell (for example as described in U.S. Pat. No. 8,130,548 toWidjaj a et al., titled “Semiconductor Memory Having Floating BodyTransistor and Method of Operating” (“Widjaja-1”), U.S. Pat. No.8,077,536, “Method of Operating Semiconductor Memory Device withFloating Body Transistor Using Silicon Controlled Rectifier Principle”(“Widjaja-2”), U.S. Patent Application Publication No. 2013/0264656A113/746,523, “Memory Device Having Electrically Floating BodyTransistor” (“Widjaja-3”), all of which are hereby incorporated herein,in their entireties, by reference thereto). This bi-stability isachieved due to the applied back bias which causes impact ionization andgenerates holes to compensate for the charge leakage current andrecombination.

SUMMARY OF THE INVENTION

A semiconductor memory cell comprising an electrically floating bodyhaving two stable states is disclosed. A method of operating the memorycell is disclosed.

In one aspect of the present invention, a semiconductor memory instanceis provided that includes an array of memory cells, including aplurality of semiconductor memory cells arranged in at least one columnand at least one row. Each semiconductor memory cell includes a floatingbody region configured to be charged to a level indicative of a state ofthe memory cell; a plurality of buried well regions, wherein each of theburied well regions can be individually selected; and a decoder circuitto select at least one of the buried well regions.

In at least one embodiment, each memory cell is configured to provide atleast two stable states.

In at least one embodiment, each memory cell further comprises a firstregion in electrical contact with the floating body region and a secondregion in electrical contact with the floating body region.

In at least one embodiment, each memory cell further comprises a gatepositioned between the first and second regions.

In at least one embodiment, an address signal is provided as an input tothe decoder circuit to select the buried well region.

In at least one embodiment, a bias to one or more of the buried wellregions may be removed, while maintaining bias to others of the buriedwell regions.

In at least one embodiment, a signal generator circuit is provided tosupply bias conditions for operations of the array.

In at least one embodiment, the signal generator circuit providesdifferent ramp rates for read and write operations.

In at least one embodiment, the ramp rates for the read operations arelower than the ramp rates for the write operations.

In another aspect of the present invention, a semiconductor memoryinstance includes: an array of semiconductor memory cells, the arraycomprising at least one memory sub-array, each memory sub-arraycomprising: a plurality of the semiconductor memory cells arranged in atleast one column and at least one row, each the semiconductor memorycell comprising: a floating body region configured to be charged to alevel indicative of a state of the semiconductor memory cell; a buriedwell region; and a decoder circuit to select at least one of the atleast one memory sub-array.

In at least one embodiment, at least one of the at least one memorysub-array may be selectively disabled.

In at least one embodiment, a bias to the buried well region within oneof the at least one memory sub-array may be applied to maintain thestates of the semiconductor memory cells in the one of the at least onememory sub-array during a high portion of a clock cycle and turned-offduring a low portion of the clock cycle.

In at least one embodiment, each semiconductor memory cell is configuredto provide at least two stable states.

In another aspect of the present invention, an integrated circuit deviceincludes an array of semiconductor memory cells, the array comprising: aplurality of the semiconductor memory cells arranged in at least onecolumn and at least one row, each semiconductor memory cell comprising:a floating body region configured to be charged to a level indicative ofa state of the semiconductor memory cell, respectively; a plurality ofburied well regions, wherein each buried well region can be individuallyselected; and a decoder circuit to select at least one of the buriedwell regions.

In at least one embodiment, the integrated circuit device furtherincludes a supply generator circuitry.

In at least one embodiment, each semiconductor memory cell is configuredto provide at least two stable states.

In at least one embodiment, the integrated circuit device furtherincludes an address signal as an input to the decoder circuit to selectat least one of the buried well regions.

In at least one embodiment, a bias to at least one of the buried wellregions may be removed, while maintaining bias to at least one other ofthe buried well regions.

In at least one embodiment, the integrated circuit device furtherincludes a signal generator circuit to provide bias conditions foroperations of the array.

In at least one embodiment, the signal generator circuit providesdifferent ramp rates for read and write operations.

These and other features of the invention will become apparent to thosepersons skilled in the art upon reading the details of the devices andmethods as more fully described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram for a memory instance according to anembodiment of the present invention.

FIG. 1B is a schematic layout view of a prior art 6-transistor SRAMmemory cell.

FIG. 1C schematically illustrates a layout view of a memory arrayaccording to an embodiment of the present invention.

FIGS. 2 and 3 schematically illustrate cross-sectional views of a memorycell according to an embodiment of the present invention.

FIG. 4A is a schematic, cross-sectional illustration of a memory cellaccording to another embodiment of the present invention.

FIG. 4B is a schematic, top-view illustration of the memory cell shownin FIG. 4A.

FIGS. 4C and 4D schematically illustrate a layout view of a memory arrayaccording to an embodiment of the present invention.

FIGS. 5 and 6 schematically illustrate equivalent circuitrepresentations of the memory cells shown in FIGS. 2-4.

FIG. 7 schematically illustrates multiple cells of the type shown inFIGS. 2-4 joined to make a memory array, according to an embodiment ofthe present invention.

FIG. 8A schematically illustrates a holding operation performed on amemory array according to an embodiment of the present invention.

FIG. 8B illustrates exemplary bias conditions applied on the terminalsof a memory cell of the array of FIG. 8A.

FIG. 9A shows an energy band diagram characterizing an intrinsic bipolardevice when a floating body region is positively charged and a positivebias is applied to a buried well region of a memory cell according to anembodiment of the present invention.

FIG. 9B shows an energy band diagram of an intrinsic bipolar device whena floating body region is neutrally charged and a positive bias isapplied to a buried well region of a memory cell according to anembodiment of the present invention.

FIG. 10 shows a graph of the net current I flowing into or out of afloating body region as a function of the potential V of the floatingbody, according to an embodiment of the present invention.

FIG. 11 shows a schematic curve of a potential energy surface (PES) of amemory cell according to an embodiment of the present invention.

FIG. 12 illustrates a charge stored in a floating body region of amemory cell as a function of a potential applied to a buried wellregion, connected to a BW terminal, according to an embodiment of thepresent invention.

FIG. 13A illustrates memory array segmentation with multiple memorysub-arrays. The BW terminals may be controlled independently for eachmemory sub-array according to an embodiment of the present invention.

FIG. 13B illustrates an exemplary BW terminal decoder circuitry,according to an embodiment of the present invention.

FIG. 14 schematically illustrates a read operation performed on a memoryarray according to an embodiment of the present invention.

FIG. 15 illustrates bias conditions applied on the terminals of aselected memory cell to perform a read operation, according to anembodiment of the present invention.

FIG. 16 schematically illustrate an equivalent capacitor circuitrepresentation of the memory cells shown in FIGS. 2-4.

FIG. 17 shows a schematic map of word line voltage and bit line voltagethat enables a floating body potential to be higher than a transitionvoltage for write logic-1, according to an embodiment of the presentinvention.

FIGS. 18A and 18B schematically illustrate a write logic-1 operationperformed on a memory array according to an embodiment of the presentinvention.

FIGS. 19A and 19B schematically illustrate a write logic-1 operationperformed on a memory array according to another embodiment of thepresent invention.

FIG. 20 shows a schematic map of word line voltage and bit line voltagethat enables a floating body potential to be lower than a transitionvoltage for write logic-0, according to an embodiment of the presentinvention.

FIG. 21 schematically illustrates a write logic-0 operation performed ona memory array according to an embodiment of the present invention.

FIG. 22 illustrates bias conditions applied on the terminals of aselected memory cell to perform a write logic-0 operation, according toan embodiment of the present invention.

FIG. 23 schematically illustrates a write logic-0 operation performed ona memory array according to another embodiment of the present invention.

FIG. 24 illustrates bias conditions applied on the terminals of aselected memory cell to perform a write logic-0 operation according toanother embodiment of the present invention.

FIG. 25 schematically illustrates a write logic-0 operation performed ona memory array according to another embodiment of the present invention.

FIG. 26 illustrates bias conditions applied on the terminals of aselected memory cell to perform a write logic-0 operation according toanother embodiment of the present invention.

FIG. 27 shows a schematic map of word line voltage and bit line voltagefor holding logic-1 states, according to an embodiment of the presentinvention.

FIG. 28 schematically illustrates a holding operation performed on amemory array according to another embodiment of the present invention.

FIGS. 29A and 29B are schematic, top-view illustrations of the memorycell shown in FIGS. 2-4 having increased capacitive coupling fromsource/drain regions to the floating body region.

FIG. 30 schematically illustrates a layout view of a memory cellaccording to another embodiment of the present invention.

FIGS. 31-34 schematically illustrate lithography steps to form thememory cell shown in FIGS. 1C and 2-4 according an embodiment of thepresent invention.

FIGS. 35-37 schematically illustrate multiple cells of the type in FIGS.2-4 joined in an array according to another embodiment of the presentinvention.

FIG. 38 schematically illustrates a read operation performed on thememory array shown in FIGS. 35-37 according to an embodiment of thepresent invention.

FIGS. 39-40 schematically illustrate self-reference read operationscheme according to an embodiment of the present invention.

FIGS. 41-42 are schematic, cross-sectional illustrations of a verticalchannel memory cell according to another embodiment of the presentinvention.

FIG. 43 illustrates an equivalent circuit representation of the memorycell shown in FIGS. 41-42.

FIG. 44 illustrates bias conditions applied on the terminals of a memorycell to perform a holding operation, according to an embodiment of thepresent invention.

FIG. 45A shows an energy band diagram characterizing an intrinsicbipolar device when a floating body region is positively charged and apositive bias is applied to a buried well region of a memory cellaccording to an embodiment of the present invention.

FIG. 45B shows an energy band diagram of an intrinsic bipolar devicewhen a floating body region is neutrally charged and a positive bias isapplied to a buried well region of a memory cell according to anembodiment of the present invention.

FIG. 46 illustrates bias conditions applied on the terminals of a memorycell to perform a read operation, according to an embodiment of thepresent invention.

FIGS. 47 and 48 illustrate bias conditions applied on the terminals of amemory cell to perform a write logic-1 operation, according to anembodiment of the present invention.

FIGS. 49 and 50 illustrate bias conditions applied on the terminals of amemory cell to perform a write logic-0 operation, according to anembodiment of the present invention.

FIG. 51 is a schematic, top-view illustration of a multi-timeprogrammable (MTP) memory cell according to another embodiment of thepresent invention.

FIGS. 52A-52B are schematic, cross-sectional illustrations of an MTPmemory cell shown in FIG. 51.

FIG. 52C illustrates an equivalent circuit representation of a memorycell of FIGS. 52A-52B.

DETAILED DESCRIPTION OF THE INVENTION

Before the present devices and methods are described, it is to beunderstood that this invention is not limited to particular embodimentsdescribed, as such may, of course, vary. It is also to be understoodthat the terminology used herein is for the purpose of describingparticular embodiments only, and is not intended to be limiting, sincethe scope of the present invention will be limited only by the appendedclaims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassedwithin the invention. The upper and lower limits of these smaller rangesmay independently be included or excluded in the range, and each rangewhere either, neither or both limits are included in the smaller rangesis also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications mentioned herein areincorporated herein by reference to disclose and describe the methodsand/or materials in connection with which the publications are cited.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “aterminal” includes a plurality of such terminals and reference to “thecell” includes reference to one or more cells and equivalents thereofknown to those skilled in the art, and so forth.

The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the present application. Thedates of publication provided may be different from the actualpublication dates which may need to be independently confirmed.

FIG. 1A illustrates a memory instance 1200, comprising memory array 100and periphery circuitries associated with the memory array 100. Examplesof the periphery circuitries are shown in FIG. 1A: control logic 102which receives for example enable (/E) and write (/W) signals andcontrols the operation of the memory array; address buffer 110, whichtransmits the address received to row decoder 112 and column decoder114; reading circuitry such as sense amplifier 116 and error correctioncircuit (ECC) 118; data buffer 120, which outputs the read data ortransmits the write data into write drivers 125; analog supplygenerators and/or regulators 135 which provide additional voltage levelsneeded for the memory array operation; redundancy logic 145 which may beused to increase the yield of the memory instance; built-in-self-test(BIST) 155 which may be used to set the trim levels for the supplygenerators 135 and/or replace the defective units with redundant array.The memory instance 1200 may be a discrete memory component or it may beembedded inside another integrated circuit device 1000.

FIG. 1B is a layout view of a six-transistor SRAM cell 2000 (for exampleas described in “Embedded Memories for Nano-Scale VLSI, K. Zhang (ed.),p. 42). The SRAM unit cell (the basic repeating cell) 2002 is showninside the dashed line. The SRAM unit cell 2002 comprises 4 n-typetransistors 2004 and 2 p-type transistors 2006. The transistors 2004,2006 are defined by the DIFF layer and the POLY gate. The DIFF layerrepresents the active area of the transistor (which typically covers thechannel region and the source and drain junctions) while the POLY layerrepresents the region of the materials forming the gate electrode. Thearea where DIFF and POLY intersects defines the channel region as wellas the gate region of the transistors. The POLY layers defining then-type transistors are labeled as N1, N4, N5, and N6, while the POLYlayers defining the p-type transistors are labeled as P1 and P2. As theexample in FIG. 1B illustrates, the transistor width (defined by thewidth of the DIFF layer) varies between the n-type and p-typetransistors. The width of the n-type transistors may also be differentin another SRAM design to improve the stability and performance of theSRAM cell.

FIG. 1C illustrates a layout view of an exemplary memory array 100according to an embodiment of the present invention. The exemplarymemory array 100 in FIG. 1C comprises two rows and two columns, whereone direction (for example the row direction) is defined by the POLYlayers 160 and another direction (for example the column direction) isdefined by the long direction of the DIFF layers 130 (or the METAL2layers 146 in FIG. 4D). There are four memory cells 50 shown in FIG. 1Cand the unit cell (the basic repeating cell) is enclosed in dashedlines. Also shown in the layout view of FIG. 1C are CONTACT layer 140,source and drain regions 16 and 18, and floating body region 24 as wellas buried well layer 170.

Referring to FIG. 2, a memory cell 50 according to an embodiment of thepresent invention is shown. A plurality of memory cells 50 constitutememory array 100 as shown in FIG. 1C. Memory cell 50 includes asubstrate 12 of a first conductivity type such as p-type, for example.Alternatively, the first conductivity type can be n-type. Substrate 12is typically made of silicon, but may also comprise, for example,germanium, silicon germanium, gallium arsenide, carbon nanotubes, and/orother semiconductor materials. In some embodiments of the invention,substrate 12 can be the bulk material of the semiconductor wafer. Inanother embodiment shown in FIG. 3, substrate 12A of a firstconductivity type (for example, p-type) can be a well of the firstconductivity type embedded in a well 29 of the second conductivity type,such as n-type. The well 29 in turn can be another well inside substrate12B of the first conductivity type (for example, p-type). In anotherembodiment, well 12A can be embedded inside the bulk of thesemiconductor wafer of the second conductivity type (for example,n-type), where region 29 represents bulk semiconductor substrate havingsecond conductivity type. These arrangements allow for segmentation ofthe substrate terminal, which is connected to region 12A. To simplifythe description, the substrate 12 will usually be drawn as thesemiconductor bulk material as it is in FIG. 2.

Memory cell 50 also includes a buried layer region 22 of a secondconductivity type, such as n-type, for example (or p-type, when thefirst conductivity type is n-type); a floating body region 24 of thefirst conductivity type, such as p-type, for example; and source/drainregions 16 and 18 of the second conductivity type, such as n-type, forexample.

Buried layer 22 may be formed by an ion implantation process on thematerial of substrate 12. Alternatively, buried layer 22 can be grownepitaxially on top of substrate 12 or formed through a solid statediffusion process.

The floating body region 24 of the first conductivity type is bounded ontop by source line region 16, drain region 18, and insulating layer 62(or by surface 14 in general), on the sides by insulating layer 26, andon the bottom by buried layer 22. Floating body 24 may be the portion ofthe original substrate 12 above buried layer 22 if buried layer 22 isimplanted. Alternatively, floating body 24 may be epitaxially grown.Depending on how buried layer 22 and floating body 24 are formed,floating body 24 may have the same doping as substrate 12 in someembodiments or a different doping, if desired in other embodiments.

A source line region 16 having a second conductivity type, such asn-type, for example (or p-type, when the first conductivity type isn-type), is provided in floating body region 24, so as to bound aportion of the top of the floating body region in a manner discussedabove, and is exposed at surface 14. Source line region 16 may be formedby an implantation process on the material making up substrate 12,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion or a selective epitaxialgrowth process could be used to form source line region 16.

A bit line region 18, also referred to as drain region 18, having asecond conductivity type, such as n-type, for example (or p-type, whenthe first conductivity type is n-type), is also provided in floatingbody region 24, so as to bound a portion of the top of the floating bodyregion in a manner discussed above, and is exposed at cell surface 14.Bit line region 18 may be formed by an implantation process on thematerial making up substrate 12, according to any implantation processknown and typically used in the art. Alternatively, a solid statediffusion or a selective epitaxial growth process could be used to formbit line region 18.

A gate 60 is positioned in between the source line region 16 and thedrain region 18, above the floating body region 24. The gate 60 isinsulated from the floating body region 24 by an insulating layer 62.Insulating layer 62 may be made of silicon oxide and/or other dielectricmaterials, including high-K dielectric materials, such as, but notlimited to, tantalum peroxide, titanium oxide, zirconium oxide, hafniumoxide, and/or aluminum oxide. The gate 60 may be made of, for example,polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and their nitrides.

Insulating layers 26 (like, for example, shallow trench isolation(STI)), may be made of silicon oxide, for example, though otherinsulating materials may be used. Insulating layers 26 insulate memorycell 50 from adjacent memory cell 50. The bottom of insulating layer 26may reside inside the buried region 22 allowing buried region 22 to becontinuous as shown in FIGS. 2 and 3. Alternatively, the bottom ofinsulating layer 26 may reside below the buried region 22 as in FIGS. 4Aand 4B (shown better in FIG. 4A). This requires a shallower insulatinglayer 28, which insulates the floating body region 24, but allows theburied layer 22 to be continuous in the perpendicular direction of thecross-sectional view shown in FIG. 4A. For simplicity, only memory cell50 with continuous buried region 22 in all directions will be shown fromhereon.

Cell 50 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to bit line region 18, source line (SL) terminal72 electrically connected to source line region 16, buried well (BW) ordeep n-well (DNWL) terminal 76 electrically connected to buried layer22, and substrate terminal 78 electrically connected to the substrate12. Alternatively, the SL terminal 72 may be electrically connected toregion 18 and BL terminal 74 may be electrically connected to region 16.

FIG. 4C further illustrates the conductive or metal lines which formconnections to the terminals of the memory cells 50 in the array 100.METAL1 layers 142 form metal lines which are connected to thesource/drain regions 16 and 18 through the CONTACT layers 140 (locatedunderneath the METAL1 layers 142 and is not shown in FIG. 4C). Alsoshown in FIG. 4C are VIA1 layers 144, which later form connectionsbetween METAL1 layer and METAL2 layer 146 as shown in FIG. 4D. Asillustrated in FIG. 4D, the direction of METAL1 layer 142 and POLY layer160 define one direction of the memory array (for example the rowdirection) and METAL2 layer 146 defines another direction of the memoryarray (for example the column direction).

FIG. 5 illustrates an equivalent circuit representation of memory cell50. Inherent in memory cell 50 are metal-oxide-semiconductor (MOS)transistor 20, formed by source line region 16, gate 60, bit line region18, and floating body region 24, and bipolar devices 30 a and 30 b,formed by buried well region 22, floating body region 24, and sourceline region 16 or bit line region 18, respectively.

Also inherent in memory device 50 is bipolar device 30 c, formed bysource line region 16, floating body 24, and bit line region 18. Fordrawings clarity, bipolar device 30 c is shown separately in FIG. 6.

FIG. 7 schematically illustrates an exemplary embodiment of a memoryarray 100 of memory cells 50 (four exemplary instances of memory cell 50being labeled as 50 a, 50 b, 50 c and 50 d) arranged in rows andcolumns. In many, but not all, of the figures where array 100 appears,representative memory cell 50 a will be representative of a “selected”memory cell 50 when the operation being described has one (or more insome embodiments) selected memory cell(s) 50. In such figures,representative memory cell 50 b will be representative of an unselectedmemory cell 50 sharing the same row as selected representative memorycell 50 a, representative memory cell 50 c will be representative of anunselected memory cell 50 sharing the same column as selectedrepresentative memory cell 50 a, and representative memory cell 50 dwill be representative of an unselected memory cell 50 sharing neither arow or a column with selected representative memory cell 50 a.

Several operations can be performed by memory cell 50 such as holding,read, write logic-1 and write logic-0 operations, and have beendescribed in U.S. Patent Application Publication No. 2010/00246284 toWidjaja et al., titled “Semiconductor Memory Having Floating BodyTransistor and Method of Operating” (“Widjaja-1”) and U.S. PatentApplication Publication No. 2010/0034041, “Method of OperatingSemiconductor Memory Device with Floating Body Transistor Using SiliconControlled Rectifier Principle” (“Widjaja-2”), which are both herebyincorporated herein, in their entireties, by reference thereto.

FIG. 8A schematically illustrates performance of a holding operation onmemory array 100, while FIG. 8B shows an example of bias conditionsapplied on the terminals of a memory cell 50 during a holding operation,according to an exemplary, non-limiting embodiment. A holding operationis performed by applying a positive back bias to the BW terminal 76,zero or negative bias on the WL terminal 70, zero bias on the BLterminal 74, SL terminal 72, and substrate terminal 78. Alternatively,the substrate terminal 78 may be left floating. In another embodiment,one of the SL terminal 72 or BL terminal 74 may be left floating. Thepositive back bias applied to the buried layer region 22 connected tothe BW terminal 76 will maintain the state of the memory cell 50 that itis connected to. The positive bias applied to the BW terminal 76 needsto generate an electric field sufficient to trigger an impact ionizationmechanism when the floating body region 24 is positively charged, aswill be described through the band diagrams shown in FIGS. 9A and 9B.The impact ionization rate as a function of the electric field is forexample described in “Physics of Semiconductor Devices”, Sze S. M. andNg K. K. (“Sze”), which is hereby incorporated herein, in its entirety,by reference thereto.

In one embodiment the bias conditions for the holding operation onmemory cell 50 are: 0 volts is applied to WL terminal 70, 0 volts isapplied to BL terminal 74, 0 volts is applied to SL terminal 72, apositive voltage, for example, +1.2 volts is applied to BW terminal 76,and 0 volts is applied to the substrate terminal 78. In otherembodiments, different voltages may be applied to the various terminalsof memory cell 50 and the exemplary voltages described are not limiting.

FIG. 9A shows an energy band diagram characterizing the intrinsic n-p-nbipolar device 30 b when the floating body region 24 is positivelycharged and a positive bias voltage is applied to the buried well region22. The vertical dashed lines mark the different regions of the bipolardevice 30 b. The energy band diagram of the intrinsic n-p-n bipolardevice 30 a can be constructed in a similar manner, with the source lineregion 16 (connected to the SL terminal 72) in place of the bit lineregion 18 (connected to the BL terminal 74). The horizontal dashed linesindicate the Fermi levels in the various regions of the n-p-n transistor30 b. The Fermi level is located in the band gap between the solid line27 indicating the top of the valence band (the bottom of the band gap)and the solid line 29 indicating the bottom of the conduction band (thetop of the band gap) as is well known in the art. If floating body 24 ispositively charged, a state corresponding to logic “1”, the bipolartransistors 30 a and 30 b will be turned on as the positive charge inthe floating body region lowers the energy barrier of electron flow(from the source line region 16 or bit line region 18) into the baseregion (floating body region 24). Once injected into the floating bodyregion 24, the electrons will be swept into the buried well region 22(connected to BW terminal 76) due to the positive bias applied to theburied well region 22. As a result of the positive bias, the electronsare accelerated and create additional hot carriers (hot hole and hotelectron pairs) through an impact ionization mechanism. The resultinghot electrons flow into the BW terminal 76 while the resulting hot holeswill subsequently flow into the floating body region 24. When thefollowing condition is met: β×(M−1)≈1—where β is the forwardcommon-emitter current gain of the bipolar transistors 30 a or 30 b andM is the impact ionization coefficient—the amount of holes injected intothe floating body region 24 compensates for the charge lost due to p-njunction forward bias current between the floating body region 24 andthe source line region 16 or bit line region 18 and due to holesrecombination. This process maintains the charge (i.e. holes) stored inthe floating body region 24 which will keep the n-p-n bipolartransistors 30 a and 30 b on for as long as a positive bias is appliedto the buried well region 22 through BW terminal 76.

The region where the product β×(M−1) approaches 1 and is characterizedby hole current moving into the base region of a bipolar transistor issometimes referred to as the reverse base current region and has beendescribed for example in “A New Static Memory Cell Based on Reverse BaseCurrent (RBC) Effect of Bipolar Transistor”, K. Sakui et al., pp. 44-47,International Electron Devices Meeting, 1988 (“Sakui-1”), “A New StaticMemory Cell Based on the Reverse Base Current Effect of BipolarTransistors”, K. Sakui et al., pp. 1215-1217, IEEE Transactions onElectron Devices, vol. 36, no. 6, June 1989 (“Sakui-2”), “On BistableBehavior and Open-Base Breakdown of Bipolar Transistors in the AvalancheRegime—Modeling and Applications”, M. Reisch, pp. 1398-1409, IEEETransactions on Electron Devices, vol. 39, no. 6, June 1992 (“Reisch”),all of which are hereby incorporated herein, in their entireties, byreference thereto.

The latching behavior based on the reverse base current region has alsobeen described in a biristor (i.e. bi-stable resistor) for example in“Bistable resistor (Biristor)—Gateless Silicon Nanowire Memory”, J.-W.Han and Y.-K. Choi, pp. 171-172, 2010 Symposium on VLSI Technology,Digest of Technical Papers, 2010 “(“J.-W. Han”), which is herebyincorporated herein, in its entirety, by reference thereto. In atwo-terminal biristor device, a refresh operation is still required.J.-W. Han describes a 200 ms data retention for the silicon nanowirebiristor memory. In memory cell 50, the state of the memory cell ismaintained due to the vertical bipolar transistors 30 a and 30 b, whilethe remaining cell operations (i.e. read and write operations) aregoverned by the lateral bipolar transistor 30 c and MOS transistor 20.Hence, the holding operation does not require any interruptions to thememory cell 50 access.

If floating body 24 is neutrally charged (the voltage on floating body24 being equal to the voltage on grounded bit line region 18), a statecorresponding to logic-0, no (or low) current will flow through then-p-n bipolar devices 30 a and 30 b. The bipolar devices 30 a and 30 bwill remain off and no impact ionization occurs. Consequently memorycells in the logic-0 state will remain in the logic-0 state.

FIG. 9B shows an energy band diagram of the intrinsic bipolar device 30a when the floating body region 24 is neutrally charged and a biasvoltage is applied to the buried well region 22. In this state theenergy level of the band gap bounded by solid lines 27A and 29A isdifferent in the various regions of n-p-n bipolar device 30 b. Becausethe potential of the floating body region 24 and the bit line region 18is equal, the Fermi levels are constant, resulting in an energy barrierbetween the bit line region 18 and the floating body region 24. Solidline 23 indicates, for reference purposes, the energy barrier betweenthe bit line region 18 and the floating body region 24. The energybarrier prevents electron flow from the bit line region 18 (connected toBL terminal 74) to the floating body region 24. Thus the n-p-n bipolardevice 30 a and 30 b will remain off.

Sakui-1 and Sakui-2 describe a memory cell based on the reverse basecurrent effect, where the base of a n-p-n bipolar transistor isconnected to a p-type MOS transistor. Reisch describes the challengeswith the memory cell described in Sakui-1 and Sakui-2, which includesthe requirement for the current of the p-type MOS transistor. Becausethe collector terminal of the bipolar transistor also serves as thechannel of the p-type MOS transistor, any changes in operatingconditions or process conditions will affect both the bipolar transistorand the p-type MOS transistor. For example, increasing the doping levelof the collector region will improve the impact ionization efficiency.However, it will also increase the doping level of the p-type MOStransistor channel region, and reduces the drive current of the p-typeMOS transistor.

An autonomous refresh for a floating body memory, without requiring tofirst read the memory cell state, has been described for example in“Autonomous Refresh of Floating Body Cell (FBC)”, Ohsawa et al., pp.801-804, International Electron Device Meeting, 2008 (“Ohsawa”), U.S.Pat. No. 7,170,807 “Data Storage Device and Refreshing Method for Usewith Such Device”, Fazan et al. (“Fazan”), which are hereby incorporatedherein, in their entireties, by reference thereto. Ohsawa and Fazanteach an autonomous refresh method by applying periodic gate and drainvoltage pulses, which interrupt access to the memory cells beingrefreshed. In memory cell 50, more than one stable state is achievedbecause of the vertical bipolar transistors 30 a and 30 b. The read andwrite operations of the memory cell 50 are governed by the lateralbipolar transistor 30 c and MOS transistor 20. Hence, the holdingoperation does not require any interruptions to the memory cell 50access.

In the holding operation described with regard to FIG. 8A, there is noindividually selected memory cell. Rather the holding operation will beperformed at all cells connected to the same buried well terminal 76. Inaddition, the holding operation does not interrupt read or write accessto the memory cell 50.

FIG. 10 shows a graph of the net current I flowing into or out of thefloating body region 24 as a function of the potential V of the floatingbody 24 (not drawn to scale). A negative current indicates a net currentflowing into the floating body region 24, while a positive currentindicates a net current flowing out of the floating body region 24. Atlow floating body 24 potential, between 0V and V_(FB0) indicated in FIG.10, the net current is flowing into the floating body region 24 as aresult of the p-n diode formed by the floating body region 24 and theburied well region 22 being reverse biased. If the value of the floatingbody 24 potential is between V_(FB0) and V_(TS), the current will switchdirection, resulting in a net current flowing out of the floating bodyregion 24. This is because of the p-n diode, formed by the floating bodyregion 24 and the bit line region 18/source line region 16, beingforward biased as the floating body region 24 becomes increasingly morepositive. As a result, if the potential of the floating body region 24is less than V_(TS), then at steady state the floating body region 24will reach V_(FB0). If the potential of the floating body region 24 ishigher than V_(TS), the current will switch direction, resulting in anet current flowing into the floating body region 24. This is as aresult of the base current flowing into the floating body region 24being greater than the p-n diode leakage current formed by the floatingbody region 24 and the bit line region 18/source line region 16. Whenthe floating body 24 potential is higher than V_(FB1), the net currentwill be out of the floating body region 24. This is because the p-ndiode leakage current formed by the floating body region 24 and the bitline region 18/source line region 16 is once again greater than the basecurrent of the bipolar devices 30 a and 30 b.

The holding operation results in the floating body memory cell havingtwo stable states: the logic-0 state and the logic-1 state separated byan energy barrier, which are represented by V_(FB0), V_(FB1), andV_(TS), respectively. FIG. 11 shows a schematic curve of a potentialenergy surface (PES) of the memory cell 50, which shows anotherrepresentation of the two stable states resulting from applying a backbias to the BW terminal 76 (connected to the buried well region 22).

The values of the floating body 24 potential where the current changesdirection, i.e. V_(FB0), V_(FB1), and V_(TS), can be modulated by thepotential applied to the BW terminal 76. These values are alsotemperature dependent.

The holding/standby operation also results in a larger memory window byincreasing the amount of charge that can be stored in the floating body24. Without the holding/standby operation, the maximum potential thatcan be stored in the floating body 24 is limited to the flat bandvoltage V_(FB) as the junction leakage current to regions 16 and 18increases exponentially at floating body potential greater than V_(FB).However, by applying a positive voltage to substrate terminal 78, thebipolar action results in a hole current flowing into the floating body24, compensating for the junction leakage current between floating body24 and regions 16 and 18. As a result, the maximum charge V_(MC) storedin floating body 24 can be increased by applying a positive bias to thesubstrate terminal 78 as shown in FIG. 12. The increase in the maximumcharge stored in the floating body 24 results in a larger memory window.

Floating body DRAM cells described in Ranica-1, Ranica-2, Villaret, andPulicani only exhibit one stable state, which is often assigned aslogic-0 state. Villaret describes the intrinsic bipolar transistorsenhance the data retention of logic-1 state, by drawing the electronswhich otherwise would recombine with the holes stored in the floatingbody region. However, only one stable state is observed because there isno hole injection into the floating body region to compensate for thecharge leakage and recombination.

Memory array 100 may be broken/segmented into multiple sub-arrays. Forexample, the buried layer region 22 may be segmented to allow forindependent memory operation. In one embodiment of the presentinvention, if the content of a memory sub-array is no longer needed, theholding/standby operation may be terminated by removing the positivebias applied to the BW terminal 76 of that memory sub-array. FIG. 13A isa schematic illustration of the BW segmentation, where memory array 100is segmented into 4 sub-arrays. Address (ADDR) and control (CNTRL)signals are used to control the operation (and the bias conditions) ofthe BW terminal 76 of each memory sub-array. A memory sub-array may alsobe disabled for example if it has been inactive for a certain period oftime. A sector enable signal (SCTR_ENBL) may be used to selectivelyenable or disable a memory sub-array. Inactive sectors will have theirassociated SCTR_ENBL signals low. Active sectors have their associatedSCTR_ENBL signals high.

FIG. 13B illustrates an exemplary BW decoder shown in a sub-array inFIG. 13A. Address (ADDR) and control (CNTRL) signals (for example readoperation signal, write operation signal, and standby operation signal)along with a sector enable (SCTR_ENBL) signal provide inputs to the ANDgate 210. The output of the AND gate 210 (along with its complementarysignal, which is generated by the inverter gate 212) drives thetransmission gate 220. The transmission gate 220 selects betweenVBW_BIAS (for example a positive voltage such as +1.2V) and GND signalsas the output to the BW terminal 76 of the memory sub-array (BW_SCTRsignal).

As an example, to optimize power management, the SCTR_ENBL signal may begoverned by a clock signal. This circuitry would be designed to savepower during the low portion of the clock cycle, yet with optimized dutycycle to maintain data integrity during the cycle high time. In anotherembodiment of the present invention, the BW_SCTR (connected to the BWterminal 76 of the memory sub-array) needs to be set high during certainmemory access (for example read and write logic-1 operation). In anotherexample, the BW_SCTR may be set low during other memory operation (forexample write logic-0 operation).

The read and write operations of the memory cell have been described,for example, in Widjaja-1 and Widjaja-2. FIGS. 14 and 15 illustrate aread operation performed on an exemplary memory array 100 by applyingthe following bias conditions: a positive voltage is applied to theselected BW terminal 76 a, a positive voltage is applied to the selectedBL terminal 74 a, zero voltage is applied to the selected SL terminal 72a, a positive voltage applied to the selected WL terminal 70 a, whilezero voltage is applied to the substrate terminal 78. The positivevoltage applied to BL terminal 74 may be less than the positive voltageapplied to WL terminal 70, in which the difference in the thresholdvoltage of the memory cell 50 is employed to represent the state of thememory cell 50. The positive voltage applied to BL terminal 74 mayalternatively be greater than or equal to the positive voltage appliedto WL terminal 70 and may generate sufficiently high electric field totrigger the bipolar read mechanism. The unselected BL terminals willremain at zero voltage and the unselected WL terminals will remain atzero or negative voltage.

In one particular non-limiting embodiment, about 0.0 volts is applied toselected SL terminal 72 a, about +0.4 volts is applied to the selectedBL terminal 74 a, about +1.2 volts is applied to the selected WLterminal 70 a, about +1.2 volts is applied to selected BW terminal 76 a,and about 0.0 volts is applied to terminal 78. The unselected terminals74 remain at 0.0 volts and the unselected terminals 70 remain at 0.0volts. FIG. 14 shows the bias conditions for the selected memory cell 50a and unselected memory cells 50 b, 50 c, and 50 d in memory array 100.However, these voltage levels may vary.

During a read operation, the selected WL terminal 70 a (electricallyconnected to gate 60) is raised from the initial/standby condition (forexample 0.0V) to the read voltage (for example +1.2V). During the risetime of the gate 60 voltage, the surface 14 channel potential will be innon-equilibrium condition as there will be a delay for electrons fromsource and/or drain regions to drift into the channel region, forexample as described in “Substrate Response of a Floating Gate n-ChannelMOS Memory Cell Subject to a Positive Linear Ramp Rate”, Han-Sheng Leeand David Scott Lowrie, Solid-State Electronics 24(3), 1981, pp.267-273, which is hereby incorporated herein, in its entirety, byreference thereto. The coupling ratio between the gate voltage and thefloating body 24 is affected by the rise time of the gate voltage, wherea higher ramp rate will result in a higher coupling ratio between thegate 60 voltage and the floating body region 24 potential. During theread operation, the rise time of the gate voltage needs to be controlledso that the increase of the floating body 24 potential by gate 60 tofloating body 24 coupling is less than the difference between thetransition voltage and the logic-0 voltage (i.e. V_(TS)-V_(FB0)) toavoid undesired writing of memory cells 50 in logic-0 state into logic-1state. The ramp rate during the read operation may be designed to beslower than the ramp rate during the write logic-1 operation, forexample by designing the timing of the signals generated by analogsupply generation/regulation block 135 (see FIG. 1). The ramp rate maybe optimized for different process technology depending on thecapacitance between the gate region 60 and the floating body region 24.In one particular non-limiting embodiment, the ramp rate of the gate 60voltage is designed to be about +1.2V/500 ps. However, this ramp ratemay vary, while maintaining a lower ramp rate for the read operationcompared to that of the write operation. For example, the ramp rate ofthe gate 60 voltage may be in the range of about +1.2V/100 ps to +1.2V/2ns.

During the read operation, the selected BL terminal 74 a is alsoincreased from the initial/standby condition (e.g. about 0.0V) to theread voltage (e.g. about +0.4V). During the rise time of the drainregion 18, hole current (from the minority carrier of the drain region18) will flow to the floating body region 24. The hole current isrelatively small as it holes are minority carriers. Nevertheless, therise time of the BL terminal 74 a needs to be controlled so that theinjected hole can flow out of the floating body region 24 (to the sourceregion 16) to avoid undesired writing of memory cells 50 from logic-0state to logic-1 state. The ramp rate during the read operation may bedesigned to be slower than the ramp rate during the write logic-1operation, for example by designing the timing of the signals generatedby analog supply generation/regulation block 135 (see FIG. 1). The ramprate may be optimized for different process technology depending on thecapacitance between the drain region 18 and the floating body region 24.In one particular non-limiting embodiment, the ramp rate of the drainregion 18 voltage is designed to be about +0.4V/500 ps. However, thisramp rate may vary, while maintaining a lower ramp rate for the readoperation compared to that of the write operation. For example, the ramprate of the gate 60 voltage may be in the range of about +0.4V/100 ps to+0.4V/2 ns.

The minority hole current (when the drain region 18 is raised to apositive voltage) is inversely proportional to the concentration of alower-doped region of the p-n junction. The hole current can thereforebe reduced by optimizing the concentration of the lower-doped region ofthe p-n junction. This can be achieved through the optimization of theprocess steps to form the doping profile of the floating body region 24and/or the drain region 18, for example through the optimization of theion implantation dose and energy and/or the subsequent thermal processto activate the dopant. Epitaxial growth process may also be used toform the floating body region 24 and/or the drain region 18.

FIG. 16 schematically illustrates an equivalent capacitor circuitrepresentation of the memory cells shown in FIGS. 2-4. The floating bodypotential (V_(FB)) is capacitively coupled with gate oxide capacitance,source side junction capacitance, drain side junction capacitance, anddeep n-well junction capacitance. Therefore, the floating body potential(V_(FB)) can be perturbed by the WL voltage, SL voltage, BL voltage, andBW voltage. As explained in FIG. 10, if the floating body potentialV_(FB) becomes higher than V_(TS), V_(FB) will reach V_(FB1) at steadystate. If the V_(FB) becomes less than V_(TS), V_(FB) will reach V_(FB0)at steady state. When the gate oxide capacitance and the drain sidejunction capacitance are relatively larger than the deep n-well junctioncapacitance, V_(FB) is preferentially controlled by V_(WL) and V_(BL).In this case, V_(FB) can become higher than V_(TS) without impactionization process in the channel as well as V_(FB) can become lowerthan V_(TS) without supplying the forward junction current to thefloating body or flowing significantly low forward junction current tothe floating body. Therefore, the writing logic-1 and writing logic-0can be accomplished by capacitive coupling between V_(WL) and V_(BL).The writing mechanism using the capacitive coupling features that thewriting logic-1 voltage of V_(BL) does not exceed the impact ionizationthreshold voltage, which is, in case of silicon semiconductor, 1.2V andthe writing logic-0 voltage of V_(BL) is zero or slightly negativevoltage.

FIG. 17 shows a schematic map of word line voltage and bit line voltagethat enables V_(FB) to be higher than V_(TS) for writing logic-1 fordifferent V_(BW). If V_(BW) is increased, the depletion width betweendeep n-well and floating body is increased and BW junction capacitanceis decreased. Therefore, the transition voltage V_(TS) is decreased.Consequently, when V_(BW) is increased, a relatively lower V_(WL) andV_(BL) can write logic-1 by capacitive coupling, compared to V_(WL) andV_(BL) levels that are required to write logic-1 when V_(BW) isrelatively lower. At a given V_(BW), the bias conditions located to theright of the curve represents a write logic-1 bias condition.

FIGS. 18A and 18B illustrate a schematic illustration of a memory array100, showing a write logic-1 operation which may be performed byapplying the following bias conditions: a positive voltage is applied tothe selected BW terminal 76 a, a positive voltage is applied to theselected BL terminal 74 a, zero voltage is applied to the selected SLterminal 72 a, a positive voltage is applied to the selected WL terminal70 a, while zero voltage is applied to the substrate terminal 78. Thecombined capacitive coupling by the positive voltage applied to theselected BL terminal 74 a and the positive voltage applied to theselected WL terminal 70 a can be sufficiently high enough to raise theV_(FB) higher than V_(TS) so as to trigger an impact ionizationmechanism of vertical bipolar transistors 30 a and 30 b. But thepositive voltage applied to the selected BL terminal 74 a and thepositive voltage applied to the WL selected terminal 70 a may not besufficiently high to trigger an impact ionization mechanism of thelateral bipolar transistor 30 c and MOS transistor 20. The combinedcapacitive coupling by the positive voltage applied to the selected BLterminal 74 a and the zero voltage applied to the unselected WLterminals 70 n can be insufficient to write logic-1. The combinedcapacitive coupling by the zero voltage applied to the unselected BLterminal 74 n and the positive voltage applied to the selected WLterminals 70 a can be insufficient to write logic-1.

In one particular non-limiting embodiment, the selected WL terminal 70 a(electrically connected to gate 60) is increased from theinitial/standby condition (e.g. about 0.0V) to the write condition (e.g.about +0.8V). The selected BL terminal 74 a (electrically connected todrain 18) is increased from the initial/standby condition (e.g. about0.0V) to the write condition (e.g. about +0.6V). Similar to that of theread operation, the coupling ratio between the gate electrode 60 and thefloating body region 24 is a function of the ramp rate. As a result, ahigher ramp rate may assist the write logic-1 operation. However, thevoltage ramp rate of the WL terminal 70 a also has to be controlled toavoid undesired writing of unselected memory cells 50 in the selectedrow from logic-0 state to logic-1 state. Similarly, the voltage ramprate of the BL terminal 74 a may also assist the write logic-1operation. However, the voltage ramp rate of the BL terminal 74 a alsohas to be controlled to avoid undesired writing of unselected memorycells 50 in the selected column from logic-0 state to logic-1 state.

FIGS. 19A and 19B show a write logic-1 operation according to anotherembodiment of the present invention, where the following bias conditionsare applied to the memory array 100: a positive voltage is applied tothe selected BW terminal 76 a, a positive voltage is applied to theselected BL terminal 74 a, a positive voltage is applied to the selectedSL terminal 72 a, a positive voltage is applied to the selected WLterminal 70 a, while zero voltage is applied to the substrate terminal78. The combined capacitive coupling by the positive voltage applied tothe selected BL terminal 74 a, selected SL terminal 72, and the selectedWL terminal 70 a can be sufficiently high to raise the V_(FB) higherthan V_(TS) so as to trigger impact ionization mechanism of verticalbipolar transistors 30 a and 30 b.

In one particular non-limiting embodiment, the selected WL terminal 70 a(electrically connected to gate 60) is increased from theinitial/standby condition (e.g. about 0.0V) to the write condition (e.g.about +0.8V), while the selected BL terminal 74 a (electricallyconnected to drain 18) and selected SL terminal 72 a (electricallyconnected to source 16) is increased from the initial/standby condition(e.g. about 0.0V) to the write condition (e.g. about +0.3V). Asdescribed, the coupling ratio between the gate electrode 60 and thefloating body region 24 is a function of the ramp rate. As a result, ahigher ramp rate may assist the write logic-1 operation. However, thevoltage ramp rate of the WL terminal 70 a also has to be controlled toavoid undesired writing of unselected memory cells 50 in the selectedrow from logic-0 state to logic-1 state. Similarly, the voltage ramprate of the BL terminal 74 a and SL terminal 72 a may also assist thewrite logic-1 operation. However, the voltage ramp rate of the BLterminal 74 a and SL terminal 72 a also have to be controlled to avoidundesired writing of unselected memory cells 50 in the selected columnfrom logic-0 state to logic-1 state, for example by designing the timingof the signals generated by analog supply generation/regulation block135 (see FIG. 1). The ramp rate may be optimized for different processtechnology depending on the capacitance between the drain region 18 andthe floating body region 24. In one particular non-limiting embodiment,the ramp rate of the drain region 18 voltage is designed to be about+0.8V/200 ps. However, this ramp rate may vary, while maintaining alower ramp rate for the read operation compared to that of the writeoperation. For example, the ramp rate of the gate 60 voltage may be inthe range of about +0.8V/20 ps to +0.8V/2 ns.

FIG. 20 shows a schematic map of word line voltage and bit line voltagethat enables a floating body potential to be lower than a transitionvoltage for write logic-0. At given V_(BW), the writing logic-0 can beaccomplished by application of the voltage from left and down side ofthe curve. If V_(BW) is increased, the positive floating body chargedensity becomes higher due to the impact ionization of vertical bipolartransistors 30 a and 30 b. Therefore the higher forward-biased junctioncurrent and thus a more negative V_(BL) is necessary to write logic-0.When V_(BL) is highly negative such as −1V, then the forward-biasedjunction current is predominant so that the all cells connected to theBL shall be written logic-0 regardless of WL voltage. However, if V_(BL)is negative and reasonably small such as −0.2V, the gate capacitivecoupling can influence the write logic-0 process. At a fixed V_(BW),when V_(WL) is decreased, V_(FB) becomes lowered due to the capacitivecoupling. In this case, the less forward-biased junction current canpull down V_(FB) below V_(TS), which implies that a lower V_(WL) can beused to write logic-0. Conversely, when V_(WL) is increased, V_(FB)becomes increased and the same forward-biased junction current may notsufficient to pull down V_(FB) below V_(TS). Therefore, thebit-selective write logic-0 can be accomplished at fixed negative V_(BL)by choosing a V_(WL) lower than the curve for a selected row and aV_(WL) higher than the curve for the unselected row(s).

FIGS. 21 and 22 are schematic illustrations of a memory array 100 andselected memory cell 50, respectively, showing a write logic-0 operationwhich may be performed through an application of a zero or negativevoltage to the selected WL terminal 70 a, a negative voltage to selectedBL terminal 74 a, zero or a positive voltage to the selected BW terminal76 a, zero voltage to the selected SL terminal 72 a, and zero orpositive voltage to substrate terminal 78. Under these conditions, thefloating body 24 potential will decrease through capacitive couplingfrom the zero or negative voltage applied to the WL terminal 70 a. Thedecrease of floating body 24 potential facilitates the forward junctioncurrent by the negative voltage applied to the BL terminal 74, whichfacilitates pulling the floating body potential below bit transitionvoltage and thus the cessation of the impact ionization mechanism ofvertical bipolar transistors 30 a and 30 b. Despite the negative voltageapplied to the selected BL terminal 74 a, the positive voltage appliedto the unselected WL terminals 70 n can increase floating body potentialand thus the forward junction current by the negative voltage applied tothe BL terminal 74 may be insufficient to write logic-0. The capacitivecoupling from zero voltage applied to the unselected BL terminal 74 nand the zero or negative voltage applied to the selected WL terminals 70a can be insufficient to write logic-0.

The selected WL terminal 70 a (electrically connected to gate 60) isdecreased from the initial/standby condition (e.g. about 0.0V) to thewrite condition (e.g. about −0.1V). The selected BL terminal 74 a(electrically connected to drain 18) is increased from theinitial/standby condition (e.g. about 0.0V) to the write condition (e.g.about −0.2V). The unselected WL terminals 70 n (electrically connectedto gate) are increased from initial/standby condition (e.g. about 0.0V)to the write inhibit condition (e.g. about +0.3V). Similar to that ofthe read operation, the coupling ratio between the gate electrode 60 andthe floating body region 24 is a function of the ramp rate. The voltageramp rate of the unselected WL terminal 70 n also has to be controlledto avoid undesired writing of unselected memory cells 50 in theunselected row from logic-0 state to logic-1 state. The ramp rate may beoptimized for different process technology depending on the capacitancebetween the gate region 60 and the floating body region 24. In oneparticular non-limiting embodiment, the ramp rate of the gate 60 voltageis designed to be about +0.3V/200 ps. However, this ramp rate may vary,while maintaining a lower ramp compared to that of the write logic-1operation. For example, the ramp rate of the gate 60 voltage may be inthe range of about +0.3V/20 ps to +0.3V/2 ns.

FIGS. 23 and 24 illustrate a schematic illustration of a memory array100, showing a write logic-0 operation according to another embodimentof the present invention, which may be performed through an applicationof a positive voltage to the selected WL terminal 70 a, a negativevoltage to selected BL terminal 74 a, zero or a positive voltage to theselected BW terminal 76 a, zero voltage to the selected SL terminal 72a, and zero or positive voltage to substrate terminal 78. Under theseconditions, the floating body 24 potential will increase throughcapacitive coupling from the positive voltage applied to the WL terminal70. As a result of the floating body 24 potential increase and thenegative voltage applied to the BL terminal 74, the p-n junction between24 and 18 is forward-biased, evacuating any holes from the floating body24.

The selected WL terminal 70 a (electrically connected to gate 60) israised from the initial/standby condition (for example 0.0V) to thewrite logic-0 voltage (for example +1.2V). As has been described above,the coupling ratio between the gate electrode 60 and the floating bodyregion 24 is a function of the ramp rate. As a result, a higher ramprate may assist the write logic-0 operation. However, the voltage ramprate of the WL terminal 70 a also has to be controlled to avoidundesired writing of unselected memory cells 50 in the selected row fromlogic-0 state to logic-1 state. The ramp rate may be optimized fordifferent process technology depending on the capacitance between thegate region 60 and the floating body region 24. In one particularnon-limiting embodiment, the ramp rate of the gate 60 voltage isdesigned to be about +1.2V/200 ps. However, this ramp rate may vary,while maintaining a lower ramp compared to that of the write logic-1operation. For example, the ramp rate of the gate 60 voltage may be inthe range of about +1.2V/20 ps to +1.2V/2 ns.

After the write logic-0 operation is finished, the potential of the BLterminal 74 is raised from the negative voltage (e.g. about −0.2V) toits standby condition (e.g. about 0.0V). The ramp rate of the BLterminal 74 needs to be controlled to avoid undesired writing ofunselected memory cells 50 in the selected column from logic-0 state tologic-1 state as well as undesired reverting of written bit state of theselected memory cell.

FIGS. 25 and 26 illustrate a write logic-0 operation according toanother embodiment of the present invention. A slight positive voltageis applied to the selected BL terminal 74 a and SL terminal 72 a. Thisachieves a similar effect as reducing the V_(BW) as this reduces thepotential difference between the BW terminal 76 and the SL and BLterminals 72 a and 74 a, respectively. A write logic-0 can then beperformed to the selected memory cell 50 by lowering the voltage ofselected WL terminal 70 a, which will lower V_(FB) through capacitivecoupling.

In one embodiment, the following bias conditions are applied: theselected WL terminal 70 a is decreased from the initial/standbycondition (e.g. about 0.0V) to the write condition (e.g. about −0.2V).The selected SL terminal 72 a and BL terminal 74 a are increased to thewrite condition (e.g. about +0.2V) from the initial/standby condition(e.g. about 0.0V).

FIG. 27 shows a schematic map of word line voltage and bit line voltagefor holding logic-1 states. At given V_(BW), if the bias voltage isformed from left and down side of the curve, the logic-1 states turnedto logic-0 state by writing logic-0 mechanism. In practice, the BLvoltage for holding is zero in order to not flow BL current. Likewise,the WL voltage for holding tends to be zero in order to shut-off theunselected bit cells. However, if the V_(DNWL) is sufficiently decreasedwhile keeping the data retention capability, the zero voltage to WL maylose logic-1 state even without involving the forward-biased junctioncurrent because the capacitive coupling from the gate 60 is solelysufficient to pull down V_(FB) below V_(TS). However, the DNWL currentto hold logic-1 at low positive V_(DNWL) and positive V_(WL) can be lessthan that at high V_(DNWL) and zero V_(WL). With this regards, lowV_(DNWL) holding and positive V_(WL), for holding condition can beapplied for low-power stand-by mode.

FIG. 28 schematically illustrates a holding operation performed on amemory array according to another embodiment of the present invention.In one embodiment the bias conditions for the holding operation onmemory cell 50 are: +0.2 volts is applied to WL terminal 70, 0 volts isapplied to BL terminal 74, 0 volts is applied to SL terminal 72, apositive voltage, for example, +1.0 volts is applied to BW terminal 76,and 0 volts is applied to the substrate terminal 78. In otherembodiments, different voltages may be applied to the various terminalsof memory cell 50 and the exemplary voltages described are not limiting.

The source and drain capacitance may be increased to improve thecoupling of the source and drain potential to the floating bodypotential. FIGS. 29A and 29B illustrate exemplary layout views of memorycells 50 with increased source and drain capacitive coupling to thefloating body region 24. The length of the source and drain regions 16and 18 (L16 and L18) may be drawn longer than the length of the floatingbody region 24 (L24) (underneath the gate region 60). In one embodiment,the ratio of the L16 (and L18) and L24 may be greater than two. Theratio may be optimized for different process technology depending on thecapacitance between the source/drain region 16/18 and the floating bodyregion 24.

FIG. 29B illustrates another layout view of memory cells 50, where thesource/drain regions are drawn wider than the floating body region 24.

FIG. 30 illustrates another layout view of another memory array 100according to another embodiment of the present invention. The memoryarray 100 includes an additional dummy POLY layer 160D which does notoverlay a DIFF region 130 (hence being referred to as dummy layer). Thedummy layer 160D for example may be a result of restrictive design rules(which guides the layout drawing of the layers) for better lithographypatterning process. As shown in FIG. 30, the unit cell of the memorycell 50 comprises two POLY 160 regions, one 160D to define the dummyregion and another 160 to define the transistor region (overlapping withDIFF layer 130).

FIGS. 31 to 34 illustrate a lithography process using cut mask to formthe layers constructing the memory cell 50 and memory array 100. FIG. 31illustrates formation of DIFF layers 130A having regular line width andspacing. The DIFF layers 130A may be formed using any lithographytechniques including single exposures, multiple patterning techniquessuch as multiple litho and multiple etch techniques or self-aligneddouble patterning processes or directed self-assembly for example suchas described in Finders, Jo, et al. “Double patterning for 32 nm andbelow: an update”, SPIE Advanced Lithography, International Society forOptics and Photonics, 2008 and Park, Sang-Min, et al. “Sub-10 nmnanofabrication via nanoimprint directed self-assembly of blockcopolymers”, ACS nano 5.11 (2011): 8523-8531, which are herebyincorporated herein, in their entireties, by reference thereto. A cutmask layer 130B (see FIG. 32), which may also have regular line widthand spacing, is then used to cut the patterns formed using the DIFFlayers 130A resulting in the final structures shown in FIG. 33. POLYlayers 160 can then be used to define the gate regions in subsequentprocess as shown in FIG. 34. The line formation followed by line cut mayalso be employed to form the POLY layers 160 (as well as other layersused in the fabrication of the memory array 100 and the integratedcircuit 1000).

FIGS. 35 and 36 illustrate a schematic layout view of memory array 100according to another embodiment of the present invention where the DIFFlayers 130 are arranged in a staggered or zig-zag pattern. As a result,memory cells in adjacent columns do not share the same POLY layer 160.In the example illustrated in FIG. 36, the first POLY layer 160 isconnected to WL terminal 70 a and the second POLY layer 160 is connectedto WL terminal 70 b, while the first METAL2 layer 146 is connected to BLterminal 74 a and the second METAL2 layer 146 is connected to BLterminal 74 b. As can be seen, memory cells in adjacent columns (forexample memory cells 50 a and 50 b) are connected to different WLterminals (memory cells 50 a and 50 b are connected to WL terminals 70 band 70 a, respectively). This is also illustrated in the equivalentcircuit representation of the memory array 100 in FIG. 37. The memoryarray 100 shown in FIG. 37 may also be referred to as folded memoryarray architecture.

The folded memory array architecture allows the use of adjacent BL as areference. In an exemplary read operation illustrated in FIG. 38, thestate of memory cell 50 a is being sensed. To perform the readoperation, the following bias conditions are applied to the selectedmemory cell 50 a: a positive voltage is applied to the BW terminal 76 a,zero voltage is applied to the selected SL terminal 72 a, a positivevoltage (or more positive than the voltage applied to unselected WLterminals) applied to the selected WL terminal 70 a, while zero voltageis applied to the substrate terminal 78 a. The following bias conditionsare applied to the unselected memory cells: a positive voltage isapplied to the BW terminal 76, zero voltage is applied to the unselectedSL terminal 72, zero voltage is applied to the unselected BL terminal74, zero voltage (or more negative than the voltage applied to theselected WL terminal 70 a) is applied to the unselected WL terminal 70,while zero voltage is applied to the substrate terminal.

The selected BL terminal and the BL terminal directly adjacent to it,for example BL terminals 74 a and 74 b are pre-charged to a positivevoltage, for example Vdd/2. After the pre-charge operation, the chargeon the selected BL terminal 74 a may or may not be discharged dependingon the state of the memory cell 50 a. If memory cell 50 a is in logic-1state having a higher conductance, then the charge on the BL terminal 74a will be discharged through the memory cell 50 a. If memory cell 50 ais in logic-0 state having a lower conductance, then the charge on theBL terminal 74 a will be discharged slower compared to if the memorycell 50 a is in logic-1 state. Because all the memory cells connected tothe BL terminal 74 b are unselected (all the unselected WL terminals 70are turned off), the BL terminal 74 b will not be discharged through theunselected memory cells. A sensing circuit, for example a senseamplifier, can then be used to compare the charge of the BL terminals 74a and 74 b.

In one particular non-limiting embodiment, about 0.0 volts is applied tothe selected SL terminal 72 a b, about +1.2 volts is applied to theselected WL terminal 70 a, about +1.2 volts is applied to BW terminal76, and about 0.0 volts is applied to terminal 78, as illustrated inFIG. 38. The unselected SL terminals 72 remain at 0.0 volts, theunselected BL terminals 74 (other than the adjacent BL terminal 74 b)are biased at 0.0 volts, and the unselected WL terminals 70 remain at0.0 volts as illustrated in FIG. 38. The selected BL terminal 74 a andthe adjacent BL terminal 74 b are then precharged to +0.4 volts.However, these voltage levels may vary while maintaining the relativerelationships between voltage levels as generally described above.

FIG. 39 schematically illustrates a self-reference read scheme that maybe used to read the state of the memory cell 50 according to anembodiment of the present invention. In this read scheme, a set of biasconditions {V₁} is applied to the selected memory cell 50 a. A propertyof the selected memory cell 50 a, for example the drain current I_(BL1)(flowing from the BL terminal 74 a to the SL terminal 72 a) is obtained.A second set of bias conditions is then applied to the selected memorycell 50 a {V₂} and the same property of the selected memory cell 50 a isthen measured again, for example the drain I_(BL2). The change in thedrain current (I_(BL2)-I_(BL1)) due to the change in the applied biasconditions {V₂-V₁} depends on the state of the selected memory cell 50a, where (I_(BL2)-I_(BL1)) is greater when the selected memory cell 50 ais in logic-1 state compared to when the selected memory cell 50 a is inlogic-0 state. Therefore, the change in the property of the selectedmemory cell 50 a as a result of the change in the applied biasconditions may be used to sense the state of the memory cell 50.

In one particular non-limiting embodiment, the first bias conditions{V₁} are as follows: about 0.0 volts is applied to selected SL terminal72 a, about +0.4 volts is applied to the selected BL terminal 74 a,about +1.2 volts is applied to the selected WL terminal 70 a, about +1.2volts is applied to selected BW terminal 76 a, and about 0.0 volts isapplied to terminal 78. The unselected terminals 74 remain at 0.0 volts,the unselected terminals 72, and the unselected terminals 70 remain at0.0 volts. A property of the selected memory cell 50 a, for example thedrain current I_(BL1) (flowing from the BL terminal 74 a to the SLterminal 72 a) is obtained. A second set of bias conditions {V₂} is thenapplied to the selected memory cell 50 a, for example by increasing theV_(BW) applied to the selected BW terminal 76 a. In one particularnon-limiting embodiment, the voltage applied to the selected BW terminal76 a is increased to about +1.3 volts, while the same bias conditionsare applied to the other terminals: about 0.0 volts is applied toselected SL terminal 72 a, about +0.4 volts is applied to the selectedBL terminal 74 a, about +1.2 volts is applied to the selected WLterminal 70 a, and about 0.0 volts is applied to terminal 78. The sameproperty of the selected memory cell 50 a (for example the draincurrent) is then measured again I_(BL2). The change in the drain cellcurrent (I_(BL2)-I_(BL1)) is greater if the selected memory cell 50 a isin logic-1 state compared to when the selected memory cell 50 a is inlogic-0 state. The change in drain cell current may be optimized fordifferent process technology. In one particular non-limiting embodiment,almost no (or very small) cell current change (for example, less than100 nA) is observed if selected memory cell 50 a is in logic-0 state,and 5 μA cell current change is observed if selected memory cell 50 a isin logic-1 state. However, the resulting cell current change may vary,which may be a result of different bias conditions and/or the processsteps forming the memory cell 50, for example the ion implantation doseand energy forming the floating body region 24 and/or the buried region22 and the thermal annealing step. For example, less than 500 nA cellcurrent change may be observed for memory cell in logic-0 state andbetween 100 nA and 50 μA difference may be observed for memory cell inlogic-1 state.

In another embodiment, the V_(BW) is kept the same in the second biasconditions, and the bias conditions to the other terminals are changed.For example, second bias conditions may be applied as follows: about−0.1 volts is applied to the selected SL terminal 72 a, about +0.3 voltsis applied to the selected BL terminal 74 a, about +1.1 volts is appliedto the selected WL terminal 70 a, about +1.2 volts is applied to theselected BW terminal 76 a, and about 0.0 volts is applied to thesubstrate terminal 78. The drain current I_(BL) (flowing from the BLterminal 74 a to the SL terminal 72 a) is then compared. The differencein the drain cell current (I_(BL2)-I_(BL1)) is greater if the selectedmemory cell 50 a is in logic-1 state compared to when the selectedmemory cell 50 a is in logic-0 state.

FIG. 40 schematically illustrates a self-reference read scheme that maybe used to read the state of the memory cell 50 according to anotherembodiment of the present invention. In this scheme, four sets of biasconditions ({V₁}, {V₂}, {V₃}, and {V₄}) are applied to the selectedmemory cell 50 a. In this read scheme, two sets of the bias conditions,for example {V₁} and {V₃}, are used to measure the background noiseinformation. In one embodiment, in order to measure the backgroundnoise, the same bias conditions are applied to the selected andunselected cells. The other two sets of bias conditions, for example{V₂} and {V₄}, are used to measure the change in the selected cellproperties (for example the drain current I_(BL)) due to the change inthe bias conditions.

In one particular non-limiting embodiment, a first set of biasconditions {V₁} is as follows: about 0.0 volts is applied to selected SLterminal 72 a, about +0.4 volts is applied to the selected BL terminal74 a, about 0.0 volts is applied to the selected WL terminal 70 a, about+1.2 volts is applied to selected BW terminal 76 a, and about 0.0 voltsis applied to terminal 78. The unselected terminals 74 remain at 0.0volts, the unselected terminals 72, and the unselected terminals 70remain at 0.0 volts. I_(BL1) can then be measured and is a measure ofthe current contribution from the unselected cells along the selected BL74 a.

A second set of bias conditions {V₂} is as follows: about 0.0 volts isapplied to selected SL terminal 72 a, about +0.4 volts is applied to theselected BL terminal 74 a, about +1.2 volts is applied to the selectedWL terminal 70 a, about +1.2 volts is applied to selected BW terminal 76a, and about 0.0 volts is applied to terminal 78. The unselectedterminals 74 remain at 0.0 volts, the unselected terminals 72, and theunselected terminals 70 remain at 0.0 volts. The drain current I_(BL2)can then be measured.

The bias conditions applied to the memory array 100 can then bemodified. For example, the V_(BW) applied to the selected BW terminal 76may be increased in {V₃} and {V₄}. Non-limiting bias conditions {V₃} areas follows: about 0.0 volts is applied to selected SL terminal 72 a,about +0.4 volts is applied to the selected BL terminal 74 a, about 0.0volts is applied to the selected WL terminal 70 a, about +1.3 volts isapplied to selected BW terminal 76 a, and about 0.0 volts is applied toterminal 78. The unselected terminals 74 remain at 0.0 volts, theunselected terminals 72, the unselected terminals 76 remain at +1.2volts, and the unselected terminals 70 remain at 0.0 volts. I_(BL3) canthen be measured and is a measure of the current contribution from theunselected cells along the selected BL 74 a when V_(BW) is changed.

A non-limiting set of bias conditions {V₄} is as follows: about 0.0volts is applied to selected SL terminal 72 a, about +0.4 volts isapplied to the selected BL terminal 74 a, about +1.2 volts is applied tothe selected WL terminal 70 a, about +1.3 volts is applied to selectedBW terminal 76 a, and about 0.0 volts is applied to terminal 78. Theunselected terminals 74 remain at 0.0 volts, the unselected terminals72, the unselected terminals 76 remain at +1.2 volts, and the unselectedterminals 70 remain at 0.0 volts. The drain current I_(BL4) can then bemeasured.

The measured properties ((I_(BL4)-I_(BL2))−(I_(BL3)-I_(BL1))) reflectthe state of the selected memory cell 50 a while removing the backgroundnoise information due to the unselected cells along the selected BLterminal 74 a. The difference in drain currents((I_(BL4)-I_(BL2))−(I_(BL3)-I_(BL1))) is higher when the selected memorycell 50 a is in the logic-1 state compared to when the selected memorycell 50 a is in the logic-0 state. The change in drain cell current maybe optimized for different process technology. In one particularnon-limiting embodiment, almost no (or very small) cell current change(for example, less than 100 nA) is observed if selected memory cell 50 ais in logic-0 state, and 5 μA cell current change is observed ifselected memory cell 50 a is in logic-1 state. However, the resultingcell current change may vary, which may be a result of different biasconditions and/or the process steps forming the memory cell 50, forexample the ion implantation dose and energy forming the floating bodyregion 24 and/or the buried region 22 and the thermal annealing step.For example, less than 500 nA cell current change may be observed formemory cell in logic-0 state and between 100 nA and 50 μA difference maybe observed for memory cell in logic-1 state.

FIGS. 41 and 42 illustrate a vertical channel memory cell 250 accordingto another embodiment of the present invention. Memory cell 250 includesa substrate 12 of a first conductivity type such as p-type, for example(alternatively, first conductivity type could be n-type). Substrate 12is typically made of silicon, but may also comprise, for example,germanium, silicon germanium, gallium arsenide, carbon nanotubes, and/orother semiconductor materials. In some embodiments of the invention,substrate 12 can be the bulk material of the semiconductor wafer. Inanother embodiment, as shown in FIG. 42, substrate 12A of a firstconductivity type (for example, p-type) can be a well of the firstconductivity type embedded in a well 29 of the second conductivity type,such as n-type (alternatively, second conductivity type is p-type whenfirst conductivity type is n-type). The well 29 in turn can be anotherwell inside substrate 12B of the first conductivity type (for example,p-type). In another embodiment, well 12A can be embedded inside the bulkof the semiconductor wafer of the second conductivity type (for example,n-type). These arrangements allow for segmentation of the substrateterminal, which is connected to region 12A. To simplify the description,the substrate 12 will usually be drawn as the semiconductor bulkmaterial as it is in FIG. 41.

Memory cell 250 also includes a bit line region 18 of a secondconductivity type, such as n-type, for example; a floating body region24 of the first conductivity type, such as p-type, for example; a sourceline region 16 of the second conductivity type, such as n-type, forexample; and an charge injector region 22 of the second conductivitytype, such as n-type, for example.

Bit line region 18 may be formed by an ion implantation process on thematerial of substrate 12. Alternatively, bit line region 18 can be grownepitaxially on top of substrate 12 or formed through a solid statediffusion process.

The floating body region 24 of the first conductivity type is bounded ontop by source line region 16 and charge injector region 32, on the sidesby insulating layer 26 (located on a plane to the front of and behindthe floating body region 24—not shown in FIGS. 41-42), on the sides bydielectric layer 62 and gate 60, and on the bottom by bit line region18. Floating body 24 may be the portion of the original substrate 12above bit line region 18 if bit line region 18 is implanted.Alternatively, floating body 24 may be epitaxially grown. Depending onhow bit line region 18 and floating body 24 are formed, floating body 24may have the same doping as substrate 12 in some embodiments or adifferent doping, if desired in other embodiments.

A source line region 16 having a second conductivity type, such asn-type, for example, is provided in floating body region 24, so as tobound a portion of the top of the floating body region in a mannerdiscussed above, and is exposed at surface 14. Source line region 16 maybe formed by an implantation process on the material making up substrate12, according to any implantation process known and typically used inthe art. Alternatively, a solid state diffusion or a selective epitaxialgrowth process could be used to form source line region 16.

A charge injector 32 having a second conductivity type, such as n-type,for example, is also provided in floating body region 24, so as to bounda portion of the top of the floating body region in a manner discussedabove, and is exposed at cell surface 14. Charge injector region 32 maybe formed by an implantation process on the material making up substrate12, according to any implantation process known and typically used inthe art. Alternatively, a solid state diffusion or a selective epitaxialgrowth process could be used to form injector region 22

A gate 60 is positioned in between the source line region 16 and thedrain region 18, on the sides of the floating body region 24. The gate60 is insulated from the floating body region 24 by an insulating layer62. Insulating layer 62 may be made of silicon oxide and/or otherdielectric materials, including high-K dielectric materials, such as,but not limited to, tantalum peroxide, titanium oxide, zirconium oxide,hafnium oxide, and/or aluminum oxide. The gate 60 may be made of, forexample, polysilicon material or metal gate electrode, such as tungsten,tantalum, titanium and their nitrides.

Insulating layers 26 (like, for example, shallow trench isolation(STI)), may be made of silicon oxide, for example, though otherinsulating materials may be used. Insulating layers 26 insulate memorycell 250 from adjacent memory cell 250. The bottom of insulating layer26 may reside below the bit line region 18 to allow for the bit lineregion 18 to be continuous in one direction and discontinuous in theother direction. This requires a deeper insulating layer 28 (not shown),which insulates the floating body region 24, but allows the bit lineregion 18 to be discontinuous in the perpendicular direction of thecross-sectional view shown in FIGS. 41 and 42.

Cell 250 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to drain region 18, source line (SL) terminal 72electrically connected to source line region 16, charge injector (CI)terminal 86 electrically connected to charge injector region 32, andsubstrate (SUB) terminal 78 electrically connected to the substrate 12.

FIG. 43 illustrates an equivalent circuit representation of memory cell250. Inherent in cell 250 is metal-oxide-semiconductor (MOS) transistor20 a formed by source line region 16, gate 60, drain region 18, andfloating body region 24. In addition, bipolar transistor 30 a formed bysource line region 16, floating body region 24, and injector region 32is also inherent in cell 250. Similarly, MOS transistors 20 b—formed bycharge injector region 32, gate 60, drain region 18, and floating bodyregion 24—is also inherent in cell 250, as shown in FIG. 43.

In the operation of memory cell 250, the bipolar transistor 30 a and/orthe transistor 20 b is used to maintain the state stored in memory cell250, while the other transistor 20 a is used for the other operations,such as read and write operations.

FIG. 44 illustrates a holding operation being performed on a selectedmemory cell 250. The holding operation is performed by applying apositive bias to the CI terminal 76, zero or low negative bias on the WLterminal 70 to turn-off the channel region between the bit line region18 of the memory cell 250 and the injector region 22, and zero bias onthe SL terminal 72, SUB terminal 78, and BL terminal 74. The positivebias applied to the charge injector region 22 connected to the CIterminal 76 will maintain the state of the memory cell 250 that it isconnected to by maintaining the charge stored in the floating bodyregion 24. The positive bias applied to the CI terminal 76 needs togenerate an electric field sufficient to trigger an impact ionizationmechanism when the floating body region 24 is positively charged, aswill be described through the band diagram shown in FIGS. 45A and 45B.

FIG. 45A shows an energy band diagram characterizing the intrinsic n-p-nbipolar device 30 a formed by source line region 16, floating bodyregion 24, and injector region 32, when the floating body region 24 ispositively charged and a positive bias voltage is applied to the chargeinjector region 32. The vertical dashed lines mark the different regionsof the bipolar device 30 a. The horizontal dashed lines indicate theFermi levels in the various regions of the n-p-n transistor 30 a. TheFermi level is located in the band gap between the solid line 27indicating the top of the valence band (the bottom of the band gap) andthe solid line 29 indicating the bottom of the conduction band (the topof the band gap) as is well known in the art. If floating body 24 ispositively charged, a state corresponding to logic “1”, the bipolartransistor 30 a will be turned on as the positive charge in the floatingbody region 24 lowers the energy barrier of electron flow from thesource line region 16 into the base region (floating body region 24).Once injected into the floating body region 24, the electrons will beswept into the charge injector region 32 (connected to CI terminal 86)due to the positive bias applied to the charge injector region 32. As aresult of the positive bias, the electrons are accelerated and createadditional hot carriers (hot hole and hot electron pairs) through animpact ionization mechanism. The resulting hot electrons flow into theCI terminal 86 while the resulting hot holes will subsequently flow intothe floating body region 24. When the following condition is met:α×(M−1)≈1—where β is the forward common-emitter current gain of thebipolar transistor 30 a and M is the impact ionization coefficient—theamount of holes injected into the floating body region 24 compensatesfor the charge lost due to p-n junction forward bias current between thefloating body region 24 and the source line region 16 or bit line region18 and due to holes recombination. This process maintains the charge(i.e. holes) stored in the floating body region 24 which will keep then-p-n bipolar transistor 30 a on for as long as a positive bias isapplied to the charge injector region 32 through CI terminal 86.

If floating body 24 is neutrally charged (the voltage on floating body24 being equal to the voltage on grounded source line region 16), astate corresponding to logic-0, no (or low) current will flow throughthe n-p-n bipolar device 30 a. The bipolar device 30 a will remain offand no impact ionization occurs. Consequently memory cells in thelogic-0 state will remain in the logic-0 state.

FIG. 45B shows an energy band diagram of the intrinsic bipolar device 30a when the floating body region 24 is neutrally charged and a biasvoltage is applied to the charge injector region 32. In this state theenergy level of the band gap bounded by solid lines 27A and 29A isdifferent in the various regions of n-p-n bipolar device 30 a. Becausethe potential of the floating body region 24 and the source line region16 is equal, the Fermi levels are constant, resulting in an energybarrier between the source line region 16 and the floating body region24. Solid line 23 indicates, for reference purposes, the energy barrierbetween the source line region 16 and the floating body region 24. Theenergy barrier prevents electron flow from the source line region 16(connected to SL terminal 72) to the floating body region 24. Thus then-p-n bipolar device 30 a will remain off.

FIG. 46 illustrates an exemplary set of bias conditions for performing aread operation on the memory cell 250 according to an embodiment of thepresent invention. The read operation is performed by applying thefollowing bias conditions: a positive bias to the WL terminal 70, apositive bias to the BL terminal 74, zero bias to the SL terminal 72,zero or positive bias to the CI terminal 86, and zero bias to thesubstrate terminal 78. All unselected WL terminals 70 (not shown) havezero or negative bias applied, all unselected BL terminals 74 (notshown) have zero volts applied, all unselected SL terminals 72 (notshown) have zero volts applied, and all unselected CI terminals 86 havezero or positive bias applied.

In one embodiment, the bias conditions for the read operation for memorycell 250 are: about +1.2 volts is applied to WL terminal 70, about +0.4volts is applied to BL terminal 74, about 0.0 volts is applied to SLterminal 72, about +1.2 volts is applied to CI terminal 86, and about0.0 volts is applied to substrate terminal 78. In other embodiments,different voltages may be applied to the various terminals of memorycell 250 and the exemplary voltages described are not limiting. Thepositive voltage applied to BL terminal 74 may be less than the positivevoltage applied to WL terminal 70, in which the difference in thethreshold voltage of the memory cell 250 is employed to represent thestate of the memory cell 250. The positive voltage applied to BLterminal 74 may also be greater than or equal to the positive voltageapplied to WL terminal 70 and may generate sufficiently high electricfield to trigger the bipolar read mechanism.

A sensing circuit typically connected to BL terminal 74 can be used todetermine the data state of the memory cell 250. Any sensing schemeknown in the art can be used in conjunction with memory cell 250.

FIG. 47 is a schematic illustration of a memory cell 250 showingexemplary bias conditions for a write logic-1 operation on the memorycell 250 through an impact ionization mechanism, according to anembodiment of the present invention. The following bias conditions areapplied: a positive voltage is applied to the selected WL terminal 70, apositive voltage is applied to the selected BL terminal 74, zero voltageis applied to the selected SL terminal 72, zero or positive voltage isapplied to the selected CI terminal 86, and zero voltage is applied tothe substrate terminal 78. This positive voltage applied to the selectedBL terminal 74 is greater than or equal to the positive voltage appliedto the selected WL terminal 70 and may generate a sufficiently highelectric field to trigger an impact ionization mechanism.

In one particular non-limiting embodiment, about +1.2 volts is appliedto the selected WL terminal 70, about +1.2 volts is applied to theselected BL terminal 74, about 0.0 volts is applied to SL terminal 72,about 0.0 volts or +1.2 volts is applied to CI terminal 86, and about0.0 volts is applied to substrate terminal 78; while about 0.0 volts isapplied to the unselected WL terminals 70, unselected BL terminals 74,unselected SL terminals, and substrate terminal 78, and 0.0 volts or+1.2 volts is applied to unselected CI terminal 86. These voltage levelsare exemplary only and may vary from embodiment to embodiment. Thus theexemplary embodiments, features, bias levels, etc., described are notlimiting.

FIG. 48 is a schematic illustration showing bias conditions for a writelogic-1 operation using band-to-band tunneling mechanism performed onmemory cell 250 according to an embodiment of the present invention. Awrite logic-1 operation using band-to-band tunneling mechanism can beperformed by applying the following bias conditions: a negative voltageis applied to the selected WL terminal 70, a positive voltage is appliedto the selected BL terminal 74, zero voltage is applied to the selectedSL terminal 72, zero or positive voltage is applied to the selected CIterminal 86, and zero voltage is applied to the substrate terminal 78.

In one particular non-limiting embodiment, about −1.2 volts is appliedto the selected WL terminal 70, about +1.2 volts is applied to theselected BL terminal 74, about 0.0 volts is applied to selected SLterminal 72, about 0.0 volts or +1.2 volts is applied to selected CIterminal 86, and about 0.0 volts is applied to substrate terminal 78;while about 0.0 volts is applied to the unselected WL terminals 70,unselected BL terminals 74, unselected SL terminals 72, and substrateterminal 78, and 0.0 volts or +1.2 volts is applied to unselected CIterminals 86. These voltage levels are exemplary only may vary fromembodiment to embodiment. Thus the exemplary embodiments, features, biaslevels, etc., described are not limiting.

The negative bias on the gate 60 (connected to WL terminal 70) and thepositive voltage on bit line region 18 (connected to BL terminal 74)create a strong electric field (for example, about 10⁶ V/cm in silicon,as described in Sze, p. 104) between the bit line region 18 and thefloating body region 24 in the proximity of gate 60. This bends theenergy band sharply upward near the gate 60 and bit line 18 junctionoverlap region, causing electrons to tunnel from the valence band of thebit line region 18 to the conduction band of the bit line region 18,leaving holes in the valence band. The electrons which tunnel across theenergy band become the drain leakage current, while the holes areinjected into floating body region 24 and become the hole charge thatcreates the logic-1 state.

FIG. 49 is a schematic illustration showing bias conditions for a writelogic-0 operation performed on memory cell 250 according to anembodiment of the present invention. A write logic-0 operation can beperformed by applying a negative voltage bias to the selected SLterminal 72, a zero voltage bias to the WL terminal 70, zero voltagebias to the BL terminal 74, zero or positive voltage bias to the CIterminal 86, and zero voltage bias to the substrate terminal 78; whilezero voltage is applied to the unselected SL terminals 72, zero voltageis applied to the unselected BL terminals 74, zero voltage bias appliedto the unselected WL terminals 70, zero or positive bias applied to theunselected CI terminals 86, and zero voltage bias applied to thesubstrate 78. Under these conditions, the p-n junction between floatingbody 24 and source line region 16 of the selected cell 250 isforward-biased, evacuating holes from the floating body 24. All memorycells 250 sharing the same selected SL terminal 72 will be written tosimultaneously. To write arbitrary binary data to different memory cells250, a write logic-0 operation is first performed on all the memorycells to be written, followed by one or more write logic-1 operations onthe memory cells that must be written to logic-1.

In one particular non-limiting embodiment, about −1.2 volts is appliedto selected SL terminal 72, about 0.0 volts is applied to WL terminal70, about 0.0 volts is applied to BL terminal 74, about 0.0 volts or+1.2 volts is applied to CI terminal 86, and about 0.0 volts is appliedto substrate terminal 78, while zero voltage is applied to theunselected SL terminals 72, zero voltage bias applied to the unselectedWL terminals 70, zero or positive bias applied to the unselected CIterminal 86, zero voltage is applied to the unselected BL terminals 74and zero voltage bias is applied to the substrate 78. These voltagelevels are exemplary only may vary from embodiment to embodiment. Thusthe exemplary embodiments, features, bias levels, etc., described arenot limiting.

FIG. 50 is a schematic illustration showing bias conditions applied fora bit-selective write logic-0 operation performed on memory cell 250according to an embodiment of the present invention. The bit-selectivewrite logic-0 operation may be performed by applying a positive voltageto the selected WL terminal 70, a negative voltage to the selected BLterminal 74, zero voltage bias to the selected SL terminal 72, zero orpositive voltage bias to the selected CI terminal 86, and a negativevoltage to the selected substrate terminal 78; while zero voltage isapplied to the unselected WL terminals 70, zero voltage is applied tothe unselected BL terminals 74, zero voltage bias is applied to theunselected SL terminals 72, zero or positive voltage is applied to theunselected CI terminals 86, and zero voltage is applied to theunselected substrate terminals 78. Under these conditions, the floatingbody 24 potential will increase through capacitive coupling from thepositive voltage applied to the WL terminal 70. As a result of thefloating body 24 potential increase and the negative voltage applied tothe BL terminal 74, the p-n junction between floating body region 24 andbit line region 18 is forward-biased, evacuating holes from the floatingbody 24.

To reduce undesired write logic-0 disturb to other memory cells 250 in amemory array, the applied potential can be optimized as follows: if thefloating body 24 potential of state logic-1 is referred to as V_(FB1),then the voltage applied to the WL terminal 70 is configured to increasethe floating body 24 potential by V_(FB1)/2 while −V_(FB1)/2 is appliedto BL terminal 74. Additionally, either ground or a slightly positivevoltage may also be applied to the BL terminals 74 of unselected memorycells 250 that do not share the same BL terminal 74 as the selectedmemory cell 250 a, while a negative voltage may also be applied to theWL terminals 70 of unselected memory cells 250 that do not share thesame WL terminal 70 as the selected memory cell 250.

As illustrated in FIG. 50, the following exemplary bias conditions maybe applied to the selected memory cell 50 to perform a bit-selectivewrite logic-0 operation: a potential of about −0.2 volts to the selectedBL terminal 74, a potential of about +1.2 volts to the selected WLterminal 70, about 0.0 volts is applied to the selected SL terminal 72,a potential of about +1.2 volts to the CI terminal 86, about −0.2 voltsto the substrate terminal 78.

FIGS. 51 and 52A-52B illustrate a top-view and cross-sectional views,respectively, of an exemplary memory word 200, which comprises aplurality of memory cells 150 operating in a multi-time programmablemode according to another embodiment of the present invention. Twomemory cells 150 are shown in the FIGS. 51 and 52A-52B. However, amemory word 200 may comprise one memory cell 150, or more than twomemory cells 150, for example 16 memory cells 150.

As shown in FIG. 52A, memory cell 150 includes a substrate 12 of a firstconductivity type such as p-type, for example (or, alternatively,n-type). Substrate 12 is typically made of silicon, but may alsocomprise, for example, germanium, silicon germanium, gallium arsenide,carbon nanotubes, and/or other semiconductor materials. In someembodiments of the invention, substrate 12 can be the bulk material ofthe semiconductor wafer. In another embodiment, as shown in FIG. 52B,substrate 12A of a first conductivity type (for example, p-type) can bea well of the first conductivity type embedded in a well 29 of thesecond conductivity type, such as n-type. The well 29 in turn can beanother well inside substrate 12B of the first conductivity type (forexample, p-type). In another embodiment, well 12A can be embedded insidethe bulk of the semiconductor wafer of the second conductivity type (forexample, n-type). These arrangements allow for segmentation of thesubstrate terminal, which is connected to region 12A. To simplify thedescription, the substrate 12 will usually be drawn as the semiconductorbulk material as it is in FIG. 51A.

Memory cell 150 also includes a buried layer region 22 of a secondconductivity type, such as n-type, for example; a base region 24 of thefirst conductivity type, such as p-type, for example; and source/drainregions 16 and 18 of the second conductivity type, such as n-type, forexample.

Buried layer 22 may be formed by an ion implantation process on thematerial of substrate 12. Alternatively, buried layer 22 can be grownepitaxially on top of substrate 12 or formed through a solid statediffusion process.

The base region 24 is common for all memory cells 150 in the memory word200. The base region 24 of the first conductivity type is bounded on topby source line region 16, drain region 18, well-tap region 19, andinsulating layer 62 (or by surface 14 in general), on the bottom byburied layer 22, and by insulating layer 26 at the edge of the memoryword 200. Base region 24 may be the portion of the original substrate 12above buried layer 22 if buried layer 22 is implanted. Alternatively,base region 24 may be epitaxially grown. Depending on how buried layer22 and the base region 24 are formed, base region 24 may have the samedoping as substrate 12 in some embodiments or a different doping, ifdesired in other embodiments.

A source line region 16 having a second conductivity type, such asn-type, for example, is provided in base region 24, so as to bound aportion of the top of the floating body region in a manner discussedabove, and is exposed at surface 14. Source line region 16 may be formedby an implantation process on the material making up substrate 12,according to any implantation process known and typically used in theart. Alternatively, a solid state diffusion or a selective epitaxialgrowth process could be used to form source line region 16.

A bit line region 18, also referred to as drain region 18, having asecond conductivity type, such as n-type, for example, is also providedin base region 24, so as to bound a portion of the top of the floatingbody region in a manner discussed above, and is exposed at cell surface14. Bit line region 18 may be formed by an implantation process on thematerial making up substrate 12, according to any implantation processknown and typically used in the art. Alternatively, a solid statediffusion or a selective epitaxial growth process could be used to formbit line region 18.

A gate 60 is positioned in between the source line region 16 and thedrain region 18, above the base region 24. The gate 60 is insulated fromthe base region 24 by a dielectric layer 62. Dielectric layer 62 may bemade of silicon oxide and/or other dielectric materials, includinghigh-K dielectric materials, such as, but not limited to, tantalumperoxide, titanium oxide, zirconium oxide, hafnium oxide, and/oraluminum oxide. The gate 60 may be made of, for example, polysiliconmaterial or metal gate electrode, such as tungsten, tantalum, titaniumand their nitrides.

Insulating layers 26 (like, for example, shallow trench isolation(STI)), may be made of silicon oxide, for example, though otherinsulating materials may be used. Insulating layers 26 insulate memoryword 200 from adjacent memory word 200. The bottom of insulating layer26 may reside inside the buried region 22 allowing buried region 22 tobe continuous as shown in FIGS. 52A and 52B.

Cell 150 includes several terminals: word line (WL) terminal 70electrically connected to gate 60, bit line (BL) terminal 74electrically connected to bit line region 18, source line (SL) terminal72 electrically connected to source line region 16, buried well (BW)terminal 76 electrically connected to buried layer 22, and substrateterminal 78 electrically connected to the substrate 12. Alternatively,the SL terminal 72 may be electrically connected to region 18 and BLterminal 74 may be electrically connected to region 16.

Memory word 200 also comprises a well-tap region 19 of firstconductivity type, such as p-type, which is electrically connected tothe well-tap (WELL) terminal 75.

The write logic-1 operation of the memory cell 150 is performed byinducing a soft breakdown of the gate dielectric layer 62. The writeoperation may be performed for example by applying the following biasconditions: a positive voltage to the gate electrode 60, a negativevoltage to the base region 24 (through the WELL terminal 75), floatingor zero voltage to both source and drain regions 16 and 18, and zero orpositive voltage to the BW terminal 76, and zero voltage is applied tothe substrate terminal 78.

In one particular non-limiting embodiment, about +3.0 volts is appliedto the selected WL terminal 70, about −1.0 volts is applied to the WELLterminal 75, about 0.0 volts is applied to terminal SL 72, about 0.0volts is applied to the selected BL terminal 74, about +1.2 volts isapplied to terminal 76, and about 0.0 volts is applied to substrateterminal 78.

The read operation is performed by applying the following biasconditions: a positive voltage is applied to the selected WL terminal70, a positive voltage is applied to the selected BL terminal 74, zerovoltage is applied to the SL terminal 72, a positive voltage is appliedto the BW terminal 76, zero voltage is applied to the substrate terminal78, while WELL terminal 75 is left floating. A higher gate leakagecurrent will flow from the gate 60 to the base region 24 when softbreakdown has happened on gate dielectric 62 compared to if the gatedielectric 62 has not had a soft breakdown.

FIG. 52C illustrates an equivalent circuit representation of the memorycell 150, which illustrates a bipolar device 130. The base current (fromthe gate 60) will be amplified by the bipolar device 130. As a result, ahigh current may be observed flowing from the BL terminal 74 to the SLterminal 72, which may be used to determine the state of the memory cell150. A memory cell 150 where the gate dielectric 62 has had a softbreakdown will conduct a higher current (from the BL terminal 74 to theSL terminal 72). As a result, only a soft breakdown is needed, incontrast to the irreversible hard breakdown used in one-timeprogrammable memory cell, for example as described in U.S. Pat. No.6,667,902 to Jack Zezhong Peng, titled “Semiconductor memory cell andmemory array using a breakdown phenomena in an ultra-thin dielectric”and U.S. Pat. No. 7,402,855 to Wlodek Kurjanowicz, titled “Split-ChannelAntifuse Array Architecture”, both of which are hereby incorporatedherein, in their entireties, by reference thereto. The voltage appliedto the selected WL terminal 70 may be less than the threshold voltage ofthe memory cell 150 to reduce the current flow from the channel regionnear the surface 14.

In one particular non-limiting embodiment, about +0.4 volts is appliedto the selected WL terminal 70, about 0.0 volts is applied to terminalSL 72, about +1.2 volts is applied to the selected BL terminal 74, about+1.2 volts is applied to terminal 76, and about 0.0 volts is applied toterminal 78, while WELL terminal 75 is left floating.

The write logic-1 operation may also be performed by first applying apositive voltage applied to the WL terminal 70, followed by ameasurement of the bipolar current from the BL terminal 74 to the SLterminal 72, referred as verification process. If the soft breakdown isnot observed yet, a higher voltage is applied to the WL terminal 70,followed by another verification process. Once the target bipolarcurrent level is reached, the write operation is terminated. Because theWL voltage is applied gradually, this avoids the undesired hardbreakdown of the gate dielectric 62.

The soft breakdown can be recovered by applying a voltage with oppositepolarity as the write operation. This operation (which will be referredto as a reset operation or write logic-0 operation) may be performed byapplying the following bias conditions: a negative voltage is applied tothe selected WL terminal 70, zero voltage is applied to the selectedWELL terminal 75, floating or zero voltage is applied to both source anddrain regions 16 and 18, zero or positive voltage is applied to the BWterminal 76, and zero voltage is applied to the substrate terminal 78.

Memory cells 150 will still function under soft breakdown condition (incontrast to the hard breakdown of the gate dielectric 62, whicheffectively short the gate electrode 60 to the base region 24). As aresult, the reset and write operations may be performed multiple timesto the memory cells 150 and memory cells 150 may operate as multi timeprogrammable memory device.

In order to reduce the effect of a neighboring cell, the read operationmay be limited to only one selected memory cell 150 for each memory word200.

From the foregoing it can be seen that a memory cell having anelectrically floating body has been described. While the foregoingwritten description of the invention enables one of ordinary skill tomake and use what is considered presently to be the best mode thereof,those of ordinary skill will understand and appreciate the existence ofvariations, combinations, and equivalents of the specific embodiment,method, and examples herein. The invention should therefore not belimited by the above described embodiment, method, and examples, but byall embodiments and methods within the scope and spirit of the inventionas claimed.

That which is claimed is:
 1. A method of operating a semiconductormemory cell comprising: a floating body region configured to be chargedto a level indicative of a state of the memory cell; a first region inelectrical contact with said floating body region, located at a surfaceof said floating body region; a second region in electrical contact withsaid floating body region, located at a surface of said floating bodyregion, spaced apart from said first region; a gate positioned betweensaid first region and said second region; and a third region inelectrical contact with said floating body region, located below saidfloating body region; said method comprising: applying a first set ofbias conditions to said memory cell; measuring a first current: applyinga second set of bias conditions to said memory cell: measuring a secondcurrent; and comparing said first current and said current to determinesaid state of the memory cell.
 2. The method of operating asemiconductor memory cell of claim 1, further comprising applying a biasto said third region to maintain said state of the memory cell.
 3. Themethod of operating a semiconductor memory cell of claim 2, wherein saidbias to said third region is a constant positive voltage.
 4. The methodof operating a semiconductor memory cell of claim 2, wherein said biasto said third region is a periodic pulse of positive voltage.
 5. Themethod of operating a semiconductor memory cell of claim 1, wherein saidfirst current and said second current flow from said first region tosaid second region.
 6. The method of operating a semiconductor memorycell of claim 5, wherein a difference between a magnitude of said firstcurrent and a magnitude of said second current is greater when saidmemory cell is in the first state than when said memory cell is in thesecond state.
 7. The method of operating a semiconductor memory cell ofclaim 6, wherein a charge level of said floating body in said firststate is more positive than a charge level of said floating body in saidsecond state.
 8. The method of operating a semiconductor memory cell ofclaim 1, wherein said applying a first set of bias conditions comprisesapplying a first bias condition to said third region, and applying asecond set of bias conditions comprises applying a second bias conditionto said third region.
 9. The method of operating a semiconductor memorycell of claim 1, wherein said applying a first set of bias conditionscomprises applying said first set of bias conditions to said firstregion, said second region, and said gate, and said applying a secondset of bias conditions comprises applying said second set of biasconditions to said first region, said second region, and said gate. 10.The method of operating a semiconductor memory cell of claim 1, furthercomprising applying a third set of bias conditions to said memory celland applying a fourth set of bias conditions to said memory cell.